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From: "Yu, Yu-cheng" <yu-cheng.yu@intel.com>
To: Andy Lutomirski <luto@kernel.org>
Cc: X86 ML <x86@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>,
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	"open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
	Linux-MM <linux-mm@kvack.org>,
	linux-arch <linux-arch@vger.kernel.org>,
	Linux API <linux-api@vger.kernel.org>,
	Arnd Bergmann <arnd@arndb.de>,
	Balbir Singh <bsingharora@gmail.com>,
	Borislav Petkov <bp@alien8.de>,
	Cyrill Gorcunov <gorcunov@gmail.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Eugene Syromiatnikov <esyr@redhat.com>,
	Florian Weimer <fweimer@redhat.com>,
	"H.J. Lu" <hjl.tools@gmail.com>, Jann Horn <jannh@google.com>,
	Jonathan Corbet <corbet@lwn.net>,
	Kees Cook <keescook@chromium.org>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	Nadav Amit <nadav.amit@gmail.com>,
	Oleg Nesterov <oleg@redhat.com>, Pavel Machek <pavel@ucw.cz>,
	Peter Zijlstra <peterz@infradead.org>,
	Randy Dunlap <rdunlap@infradead.org>,
	"Ravi V. Shankar" <ravi.v.shankar@intel.com>,
	Vedvyas Shanbhogue <vedvyas.shanbhogue@intel.com>,
	Dave Martin <Dave.Martin@arm.com>,
	Weijiang Yang <weijiang.yang@intel.com>,
	Pengfei Xu <pengfei.xu@intel.com>
Subject: Re: [PATCH v13 8/8] x86/vsyscall/64: Fixup Shadow Stack and Indirect Branch Tracking for vsyscall emulation
Date: Mon, 28 Sep 2020 12:04:24 -0700	[thread overview]
Message-ID: <054bd574-1566-2be4-b542-884500b7319d@intel.com> (raw)
In-Reply-To: <CALCETrWuhPE3A7eWC=ERJa7i7jLtsXnfu04PKUFJ-Gybro+p=Q@mail.gmail.com>

On 9/28/2020 10:37 AM, Andy Lutomirski wrote:
> On Mon, Sep 28, 2020 at 9:59 AM Yu-cheng Yu <yu-cheng.yu@intel.com> wrote:
>>
>> On Fri, 2020-09-25 at 09:51 -0700, Andy Lutomirski wrote:
>>>> On Sep 25, 2020, at 9:48 AM, Yu, Yu-cheng <yu-cheng.yu@intel.com> wrote:
>> +
>> +               cet = get_xsave_addr(&fpu->state.xsave, XFEATURE_CET_USER);
>> +               if (!cet) {
>> +                       /*
>> +                        * This is an unlikely case where the task is
>> +                        * CET-enabled, but CET xstate is in INIT.
>> +                        */
>> +                       WARN_ONCE(1, "CET is enabled, but no xstates");
> 
> "unlikely" doesn't really cover this.
> 
>> +                       fpregs_unlock();
>> +                       goto sigsegv;
>> +               }
>> +
>> +               if (cet->user_ssp && ((cet->user_ssp + 8) < TASK_SIZE_MAX))
>> +                       cet->user_ssp += 8;
> 
> This looks buggy.  The condition should be "if SHSTK is on, then add 8
> to user_ssp".  If the result is noncanonical, then some appropriate
> exception should be generated, probably by the FPU restore code -- see
> below.  You should be checking the SHSTK_EN bit, not SSP.

The code now checks if shadow stack is on (yes, it should check SHSTK_EN 
bit, I will fix it.), then adds 8 to user_ssp.  If the result is 
canonical, then it sets the corresponding xstate.

If the resulting address is not canonical, the kernel does not know what 
the address should be either.  I think the best action to take is doing 
nothing about the shadow stack pointer, and let the application return 
and get a control protection fault.  The application should have not got 
into such situation in the first place; if it does, it should fault.

> 
> Also, can you point me to where any of these canonicality rules are
> documented in the SDM?  I looked and I can't find them.

The SDM is not very explicit.  It should have been.

> 
> This reminds me: this code in extable.c needs to change.
> 
> __visible bool ex_handler_fprestore(const struct exception_table_entry *fixup,
>                                      struct pt_regs *regs, int trapnr,
>                                      unsigned long error_code,
>                                      unsigned long fault_addr)
> {
>          regs->ip = ex_fixup_addr(fixup);
> 
>          WARN_ONCE(1, "Bad FPU state detected at %pB, reinitializing
> FPU registers.",
>                    (void *)instruction_pointer(regs));
> 
>          __copy_kernel_to_fpregs(&init_fpstate, -1);
> 
> Now that we have supervisor states like CET, this is buggy.  This
> should do something intelligent like initializing all the *user* state
> and trying again.  If that succeeds, a signal should be sent rather
> than just corrupting the task.  And if it fails, then perhaps some
> actual intelligence is needed.  We certainly should not just disable
> CET because something is wrong with the CET MSRs.
> 

Yes, but it needs more thought.  Maybe a separate patch and more discussion?

Yu-cheng


  reply	other threads:[~2020-09-28 19:04 UTC|newest]

Thread overview: 29+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-25 14:57 [PATCH v13 0/8] Control-flow Enforcement: Indirect Branch Tracking Yu-cheng Yu
2020-09-25 14:57 ` [PATCH v13 1/8] x86/cet/ibt: Add Kconfig option for user-mode " Yu-cheng Yu
2020-09-25 14:57 ` [PATCH v13 2/8] x86/cet/ibt: User-mode Indirect Branch Tracking support Yu-cheng Yu
2020-09-25 14:57 ` [PATCH v13 3/8] x86/cet/ibt: Handle signals for Indirect Branch Tracking Yu-cheng Yu
2020-09-25 14:58 ` [PATCH v13 4/8] x86/cet/ibt: ELF header parsing " Yu-cheng Yu
2020-09-25 14:58 ` [PATCH v13 5/8] x86/cet/ibt: Update arch_prctl functions " Yu-cheng Yu
2020-09-25 14:58 ` [PATCH v13 6/8] x86/vdso/32: Add ENDBR32 to __kernel_vsyscall entry point Yu-cheng Yu
2020-09-25 14:58 ` [PATCH v13 7/8] x86/vdso: Insert endbr32/endbr64 to vDSO Yu-cheng Yu
2020-09-25 16:18   ` Andy Lutomirski
2020-09-25 16:24     ` Yu, Yu-cheng
2020-09-25 14:58 ` [PATCH v13 8/8] x86/vsyscall/64: Fixup Shadow Stack and Indirect Branch Tracking for vsyscall emulation Yu-cheng Yu
2020-09-25 16:31   ` Andy Lutomirski
2020-09-25 16:47     ` Yu, Yu-cheng
2020-09-25 16:51       ` Andy Lutomirski
2020-09-28 16:59         ` Yu-cheng Yu
2020-09-28 17:37           ` Andy Lutomirski
2020-09-28 19:04             ` Yu, Yu-cheng [this message]
2020-09-29 18:37             ` Yu, Yu-cheng
2020-09-29 19:57               ` Andy Lutomirski
2020-09-29 20:00                 ` Andy Lutomirski
2020-09-30 22:33                   ` Yu, Yu-cheng
2020-09-30 23:44                     ` Andy Lutomirski
2020-10-01  1:00                       ` H.J. Lu
2020-10-01  1:10                         ` Andy Lutomirski
2020-10-01  1:21                           ` H.J. Lu
2020-10-01 16:51                           ` Yu, Yu-cheng
2020-10-01 17:26                             ` Andy Lutomirski
2020-10-06 19:09                               ` Yu, Yu-cheng
2020-10-09 17:42                                 ` Andy Lutomirski

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